The present invention relates to an analog-to-digital (A/D) conversion system provided in an electronic control system for controlling operation of an automotive engine.
Recently, motor vehicles have been equipped with an electronic control system for controlling various devices of an engine, such as fuel injectors and spark plugs, thereby improving exhaust gas emission, fuel consumption and engine power. The electronic control system controls the devices based on information represented by output signals from various sensors which detect engine operating conditions. Although sensors such as a crank angle sensor and a vehicle speed sensor generate digital signals, other sensors such as an intake air quantity sensor, a throttle position sensor and a coolant temperature sensor produce analog signals. The analog signals must be converted into digital signals so as to be processed in the electronic control system.
Japanese Patent Application Laid-Open No. 59-49351 discloses a method for converting the analog signals of the sensors into, digital signals. In that method, analog values representing an offset voltage and a reference voltage are consecutively converted to digital values before A/D conversion of the analog signals from the sensors, at least at the start of the engine control operation. A correction coefficient is calculated based on the obtained digital values of the offset voltage and the reference voltage. Digital values converted by an A/D converter, which correspond to the analog value detected by the sensors, are corrected with the correction coefficient.
However, the output signals of the sensors provided on the motor vehicle are affected a lot by various electronic noises such as spark noise generated from contacts of switches, switching noise generated when transistors go ON and OFF, spark noise in an ignition system, noises produced at the brushes of a motor and noise produced when external magnetic fields generated by starting current and alternator current cause electromagnetic induction. If analog signals containing such noise are digitized, errors occur in the control operation, causing malfunctioning of the fuel injection system, ignition system, and other systems. Since the peak voltages of the noise are relatively high, the afore-described conventional system cannot eliminate the influence of the noise.
In order to produce an accurate digital value, the noises are usually eliminated with a noise filter. However, the noise which occurs in a motor vehicle has a large peak voltage so that the time constant of the noise filter is set to a large value. As a result, the response of the filter to the analog signal consequently is delays, resulting in distortion of the waveform of the signal. Thus, the digital value obtained by the conversion does not coincide with the analog signal causing malfunctioning of the control system.